Iridium-Catalyzed Borylation of Benzene with Diboron. Theoretical Elucidation of Catalytic Cycle Including Unusual Iridium(V) Intermediate

Abstract

Iridium-catalyzed borylation of benzene with diboron was theoretically investigated with the DFT method, where an iridium(I) boryl complex, Ir(Beg)(NN) 1, and an iridium(III) tris(boryl) complex, Ir(Beg)(3)(NN) 14, (eg (ethyleneglycolato) = -OCH(2)CH(2)O-, NN = HN=CHCH=NH (diim) or 2,2'-bipyridine (bpy)) were adopted as models of active species and B(2)(eg)(2) was adopted as a model of bis(pinacolato)diboron (pinacolato = -OCMe(2)CMe(2)O-). Oxidative addition of a benzene C-H sigma-bond to 1 takes place with an activation barrier (E(a)) of 11.2 kcal/mol, followed by reductive elimination of phenylborane, Ph-Beg, from Ir(Beg)(H)(Ph)(diim) with an activation barrier of 15.6 kcal/mol. Though the oxidative addition and the reductive elimination occur with moderate activation barriers, B(2)(eg)(2) much more easily reacts with 1 to afford 14 than does benzene, of which the activation barrier is very small (2.9 kcal/mol). Oxidative addition of the benzene C-H sigma-bond to 14 occurs with a moderate activation barrier of 24.2 kcal/mol to afford an unusual seven-coordinate iridium(V) complex, Ir(H)(Ph)(Beg)(3)(bpy) 16. From this complex, phenylborane Ph-Beg is produced through the reductive elimination with concomitant formation of IrH(Beg)(2)(bpy) 17, where the activation barrier is 4.9 kcal/mol. Complex 17 further reacts with diboron to form Ir(H)(Beg)(4)(bpy) (E(a) = 8.0 kcal/mol), followed by the reductive elimination of borane H-Beg (E(a) = 2.6 kcal/mol) to regenerate Ir(Beg)(3)(bpy), when diboron exists in excess in the reaction solution. After consumption of diboron, IrH(Beg)(2)(bpy) reacts with borane, H-Beg, to form Ir(H)(2)(Beg)(3) (E(a) = 21.3 kcal/mol) followed by the reductive elimination of H(2), to regenerate Ir(Beg)(3)(bpy) with concomitant formation of H(2). Formation of the iridium(III) tris(boryl) complex 14 from IrCl(diim) and diboron was also theoretically investigated; IrCl(diim) undergoes two steps of oxidative addition of diboron to afford a seven-coordinate iridium(V) complex, IrCl(Beg)(4)(NN), from which the reductive elimination of Cl-Beg takes place easily to afford 14. From these results, it should be clearly concluded that the iridium(III) tris(boryl) complex is an active species and an unusual iridium(V) species is involved as a key intermediate in the reaction. Detailed discussion is presented on the full catalytic cycle and the importance of a seven-coordinate iridium(V) intermediate.

Iridium‐Catalyzed Borylation of Arenes and Heteroarenes via C—H Activation

“…The small steric influence of the planar dioxaboryl rings (Bpin) and an arene substrate (Ar) can also be critical for the formation of such sterically hindered hepta-coordinated Ir(V) intermediates. These processes have been supported by recent theoretical studies by Sakaki [16].…”

Theoretical and Computational Study of a Complex System Consisting of Transition Metal Element(s): How to Understand and Predict Its Geometry, Bonding Nature, Molecular Property, and Reaction Behavior

“…We theoretically investigated iridium-catalyzed direct borylation of benzene with diborane (eq 13), 114 and found that it is a little bit different from the two examples above. This reaction was experimentally performed first by Ishiyama, Hartwig, and co-workers 107,108 and also well investigated by Smith and co-workers.…”

Distortion/Interaction Analysis Reveals the Origins of Selectivities in Iridium-Catalyzed C–H Borylation of Substituted Arenes and 5-Membered Heterocycles

“…40 Compositions of molecular orbitals were calculated using the AOMix program. 41,42 ■ RESULTS AND DISCUSSION The complex lacking the four methyl groups on each Bpin, ( b p y ) I r ( B e g ) 3 ( b p y = 2 , 2 ′ -b i p y r i d i n e ; B e g = (ethyleneglycolato)boron) 9,17,43 was used as a model for (dtbpy)Ir(Bpin) 3 . Although Beg is a poorer electron donor 44 and thus expected to be less reactive in C−H oxidative addition than the Bpin-ligated catalyst, 45 the effects on regioselectivity employing Beg in place of Bpin are expected to be small (see page S5 in the SI).…”

“…This mechanism has been supported computationally. 9 Despite remarkable progress in the use of iridium catalysts, the origins of regioselectivity in this reaction are not well understood. 10 For example, in the case of substituted arenes, the regioselectivity appears to be largely controlled by steric factors.…”